Recent data implicate the K opioid system in ethanol acceptance and reinforcement during infancy and adulthood. Pharmacological blockade of K opioid receptors enhances ethanol intake and reinforcing properties in adult rodents, whereas endogenous activity at K receptors is essential for infants to find ethanol appetitive. Furthermore, activation of the K opioid system is aversive for adult rats. In contrast, in infant rats, central activation of this system is extremely reinforcing. The present proposal will test the hypothesis that the K opioid system switches from mediating appetitive to aversive properties of ethanol between the first and the second postnatal week and that this ontogenetic switch occurs when motivational properties of K opioid activation become aversive instead of appetitive. Finally, changes in the K opioid system following central ethanol administration will be assessed during the first two weeks of postnatal life.